This application claims priority from French Patent Application Serial No. FR0113488 filed on Oct. 19, 2001.
Ions sources known from prior art, as described for example in the publication xe2x80x9cElectron Opticsxe2x80x9d by Pierre Grivet, 2nd Edition, Pergamon Press Reprint, December 1971 are subject to a number of limitations.
In order to minimize the energy dispersion of ions generated in the ions source, the ionisation space is reduced.
As a result, ionisation cells may be nearly closed and the ions extraction has a rather poor yield.
In case that it is required to improve the collimating, this may be achieved by a selection through a number of narrow slits.
However introducing selection and reducing the ionisation space may drastically reduce the sensitivity to a value in the order of 10xe2x88x924 A/Torr or less, with the exception of very big instruments.
The applicant has earlier described a high intensity selective ion source in a French patent application Nr 0009081 filed Jul. 17, 2000, in which ions created by electronic bombardment in a large volume and having the same number of mass n, can be focused on a narrow slit S1 in order to obtain an emerging ion beam. In addition, U.S. application Ser. No. 09/518,507, entitled xe2x80x9cSystem for Ionization and Selective Detection in Mass Spectrometers,xe2x80x9d was filed by the inventor of the present invention on Mar. 3, 2000. This application is hereby incorporated by reference and is referred to herein as xe2x80x9cthe Evrard ""507 application.xe2x80x9d
In a first aspect the invention provides a mass spectrometer in which the ions are submitted to the action of a uniform adjustable electrical field {right arrow over (E)}1, within a set of plane parallel electrodes a1, . . . , ai, . . . an fitted with properly located slits for the ions transmission, and to the action of a uniform magnetic induction {right arrow over (B)}1. A reference system x,y is considered in a plane perpendicular to {right arrow over (B)}1, the axis x and y being respectively perpendicular and parallel to {right arrow over (E)}1, and the origin of the reference system being fixed at an average starting point of the ions. The crossed fields {right arrow over (E)}1,{right arrow over (B)}1, act together in an area where y less than d and the magnetic induction {right arrow over (B)}1 acts alone in a further area where y greater than d, d being a distance separating the average starting point of the ions from the electrode an. The selection slit S1 is located at coordinates x=2.1d and y=2d, and the value of E1 that is applied for the selection of the ions having the number of mass n is defined by             E      1        =                            2          ⁢          d                n            ⁢              e        m            ⁢              B        1        2              ,      with    ⁢          xe2x80x83        ⁢          e      m      
corresponding to a ratio charge/mass for H+. The ions are created by electronic bombardment only in the vicinity of a plane parallel to {right arrow over (B)}1, making an angle of 45xc2x0 with {right arrow over (E)}1, and a heated filament F for emitting electrons is stretched above the electrodes a1,a2, along a line where x=y, the electron beam being limited by a flat rectangular diaphragm parallel to F and located between F and the electrodes a1,a2, such that a resulting selected ion beam is parallel to the x axis when crossing S1.
In a preferred embodiment of the invention, the ions are submitted to a second couple of crossed fields {right arrow over (E)}2,{right arrow over (B)}1, at the exit S1, {right arrow over (E)}2 being created within a second set of plane parallel electrodes b1, . . . , bi, . . . bn, also fitted with properly located slits for the ions transmission. A value of the electric field E2 is equal to E1 cos 45xc2x0 and a direction of the electric field E2 makes an angle of 45xc2x0 with E1. A second selection slit S2 is located on the cycloid path on the electrode bn at a point defined by coordinates X≅8.9d and Y=0 in a further reference system that is defined by axis X and Y, wherein the X axis and the Y axis are respectively perpendicular and parallel to E2. An origin of the further reference system is fixed at S1.